Polymeric coatings having specific release properties toward adhesives are widely used. Silicones such as polydimethylsiloxanes, polymers containing predominantly dimethylsiloxane units, are frequently employed as release coatings for products such as labels or large adhesive-coated sheets sold in other than roll form. Notwithstanding a relatively high cost, such polymers are used for these applications because of their capability of providing coatings of very low release values, e.g., in the range 0.4-1.6N/100 mm width. The term "release value" refers to the amount of force required to peel a strip of pressure-sensitive adhesive tape from a surface.
Polydimethylsiloxanes are less useful, however, as release coatings on the back surface of adhesive tape, because their low release force can cause roll instability and handling problems. Such a coating on the back surface of a tape is often referred to as a low adhesion backsize or LAB. LABs for tapes in roll form ideally exhibit release values toward the adhesive in the range of about 6-35N/100 mm width. Polymers with higher release values make it increasingly difficult to use the tape, and delamination of the adhesive from the substrate often can result. Many non-silicone polymers, e.g., certain types of polyurethanes, find use as low adhesion backsizes for pressure sensitive adhesive tapes because of their much higher release value than that of the polydimethysiloxanes, typically greater than 20N/100 mm width.
For products such as tapes and liners, coatings having specific release properties toward adhesives, which are intermediate between those of the polydimethysiloxanes and conventionally used non-silicone LAB coatings have been highly desired. U.S. Pat. No. 4,728,571 (Clemens et al.) (assigned to the assignee of the present case) provides such coatings through use of polysiloxane-grafted vinyl copolymers. Controlled release is provided over a broad range of values via variation in the molecular weight and the number of polysiloxane grafts. Since the polysiloxane is chemically anchored to the backbone of the copolymers, such coatings avoid the migration problems common in prior art systems, which result in unstable release properties or in adhesive contamination. Also, since the polysiloxane constitutes only a minor weight fraction of the coating (even at low release values suitable for release liner applications), these copolymer compositions provide a potential cost savings over conventional 100% silicone release compositions and numerous blends. However, these coatings are solvent-borne.
Rising energy costs and concern over both environmental pollution and hazards to worker health have contributed to a need for development of solvent-free (or at least high solids), radiation-curable release coatings. Thus, a solventless, radiation-curable release coating composition capable of providing controlled release throughout the intermediate region suitable for LABs is highly desired. Ideally, from a cost perspective, such a composition should be low in silicone content. Most systems which have been developed, however, contain fairly high levels of silicone.
U.S. Pat. No. 4,678,846 (Weitemeyer et al.) describes acrylate or methacrylate ester modified organopolysiloxane mixtures, which can be used by themselves or in admixture with other unsaturated compounds as radiation-curable coating compositions to obtain "good adhesive properties towards adhesives."
EP 58909 (Herbefts GMBH), Published Sep. 1, 1982, discloses a radiation-curable composition containing liquid polyorganosiloxane having unsaturated groups, photosensitizer, and, optionally, vinyl monomer. The composition is used for the production of release coatings and is especially useful for coating paper.
U.S. Pat. No. 4,558,082 (Eckberg) describes photocurable acrylated silicone polymers prepared by reacting limoneneoxide-functional silicones with acrylic acid or a substituted acrylic acid in the presence of a catalyst. A preferred use of the compositions is as release coatings for paper, and it is disclosed that the compositions exhibit improved anchorage to substrates, such as supercalendered kraft paper, when up to 20% by weight of N-vinylpyrrolidone is included.
U.S. Pat. No. 4,606,933 (Griswold et al.) describes radiation-polymerizable acrylate-functional organopolysiloxanes and their use as release coatings. It is stated that it may be desirable to add a diluent to the compositions to aid in their application to a substrate. The diluent, preferably a reactive diluent, such as an acrylate ester, can be employed at levels up to about 30% by weight of the radiation-curable composition.
U.S. Pat. No. 4,783,490 (Eckberg et al.) discloses UV-curable compositions comprising mercapto-substituted silicon compounds, reactive co-compounds such as multifunctional acrylates, and photoinitiator. Reactive diluents, such as monofunctional acrylates, may optionally be added to control viscosity, although generally it is not desirable to add more than about 25% by weight. The compositions may be formulated for application to paper substrates as a release coating.
EP 159683 (DeSoto Inc.), Published Oct. 30, 1985, describes an electron beam-curable liquid release coating composition comprising from 60 to 95 parts functionalized organopolysiloxane, e.g., acrylated organopolysiloxane, from 3 to 25 parts of a polyester, e.g., a multifunctional acrylate, and from 1 to 10 parts acid, e.g., acrylic acid. The composition can optionally further include from 1 to 15 parts of a monoacrylate or monomethacrylate monomer to adjust the hardness.
U.S. Pat. No. 4,608,270 (Varaprath) discloses coating compositions comprising polydiorganosiloxanes which contain one or more acryloylamino-substituted hydrocarbon radicals.
These compositions are radiation-polymerizable to form release coatings and may optionally include polymerizable vinyl monomers.
U.S. Pat. Nos. 4,576,999 and 4,640,967 (Eckberg) describe epoxy- and/or acrylic-functional polysiloxanes which, when combined with appropriate catalysts, form ultraviolet radiation-curable release coating compositions. It is stated that cure performance and substrate adhesion may be enhanced by the addition of up to 10 parts of an aliphatic-epoxy monomer for every 10 parts epoxysilicone fluid.
U.S. Pat. No. 4,070,526 (Colquhoun et al.) discloses radiation-curable compositions comprising mercaptoalkyl-substituted polydiorganosiloxane fluid, from about 1 to 50 parts by weight vinyl monomer (per 100 parts of the fluid), and, optionally, a methylvinylpolysiloxane. Upon curing, controllably variable release of adhesives is said to be provided. Release data for compositions containing greater than 50 parts of vinyl monomer (see Table I of U.S. Pat. No. 4,070,526) indicates that release Is not reliably obtained at these higher levels, i.e., at lower levels of silicone.
A few systems have been developed which reliably provide release at low silicone levels. However, these systems suffer from the disadvantage of being inhomogeneous mixtures which, although acceptable for paper substrates, are unsuitable for use on polymeric films due to problems with dewetting. In addition, such inhomogeneous mixtures are unsuitable for use in an electrospray coating process due to problems with phase separation.
U.S. Pat. No. 4,016,333 (Gaske et al.) describes radiation-polymerizable release coating compositions (typically for paper substrates) in the form of nonaqueous emulsions of from 2 to about 50 weight percent of a liquid alkyl hydrogen polysiloxane in a radiation-polymerizable polyethylenically unsaturated liquid, preferably a polyacrylate. It is stated that an emulsifying agent can be used to promote long-term emulsion stability and that the emulsion can also be agitated as it is applied.
U.S. Pat. No. 4,201,808 (Cully et al.) discloses radiation-curable release coating compositions, most commonly for paper substrates, comprising from about 10 to 90 weight percent (based on the total weight of the composition) of an organopolysiloxane containing an average of at least one acryloxy and/or methacryloxy group per molecule, from about 90 to 10 weight percent of a low molecular weight acrylated polyol crosslinking agent, and from 0 to about 10 weight percent of a photosensitizer. To adjust viscosity, the compositions can also contain from 0.01 to about 30 weight percent of a reactive diluent, such as a liquid organic monoacrylate ester. It is stated that the composition components may undergo a degree of separation during storage, making mild agitation or mixing necessary just prior to use.
WO 88/07931 (Avery), Published Oct. 20, 1988, describes tailorable, radiation-curable release surfaces formed by curing a composition which is a dispersion of a reactive silicone (reactive group-containing dimethyl siloxane polymers present in an amount of from 1 to 30% by weight of the composition) as a discontinuous phase in a continuous phase of a reactive, resin (comprising from about 50 to 100% by weight reactive oligomer and from about 50 to 0% by weight reactive monomer based on the total weight of resin). The reactive monomer, preferably a multifunctional acrylate, is used to control viscosity. The release coatings are said to achieve-preferential concentration of the silicone at the surface by the partial or total incompatibility of the silicone and the resin. The quality of release is initially poorer and is stated to increase or improve with time.
A need exists for a solventless, radiation-curable release coating composition of substantially reduced silicone content which upon curing provides controllable, reproducible levels of release in the intermediate region suitable for LABs, which wets both paper and polymeric films well and cures to firmly anchored coatings, which has utility for a broad range of PSA types, and which does not adversely affect the tack and peel properties of PSAs with which they come in contact.
A need also exists for a coating which possesses the desired level of release immediately upon curing, thus being suitable for integrated manufacture of PSA-coated labels and tapes, e.g., wherein both the PSA and release coating are coated during the same manufacturing process.
A need also exists for a radiation-curable release coating composition which is suitable for application via an electrospray coating process.